The present invention relates generally to offshore drilling and production operations, and, more particularly, to marine drilling workover/intervention tensioning and compensating devices and methodologies.
A marine riser system may be employed to provide a conduit from a floating vessel at the water surface to the blowout preventer stack or production tree, which may be connected to the wellhead at the sea floor. A tensioning system may be utilized to maintain a variable tension in the riser string alleviating the potential for compression and, in turn, buckling or failure.
Historically, conventional riser tensioner systems have consisted of both single and dual cylinder assemblies with a fixed cable sheave at one end of the cylinder and a movable cable sheave attached to the rod end of the cylinder. The assembly is then mounted in a position on the vessel to allow convenient routing of wire rope that is connected to a point at the fixed end and strung over the movable sheaves. A hydro/pneumatic system consisting of high pressure air over hydraulic fluid applied to the cylinder forces the rod and in turn the rod end sheave to stroke out thereby tensioning the wire rope and in turn the riser.
The number of tensioner units employed is based on the tension necessary to maintain support of the riser and a percentage of overpull that is dictated by met-ocean conditions, i.e., current and operational parameters including variable mud weight, and the like.
Normal operation of these conventional type tensioning systems have required high maintenance due to the constant motion producing wear and degradation of the wire rope members. Replacing the active working sections of the wire rope by slipping and cutting raises safety concerns for personnel and has not proven cost effective. In addition, available space for installation and the structure necessary to support the units, including weight and loads imposed, particularly in deep water applications where the tension necessary requires additional tensioners, poses difficult problems for system configurations for both new vessel designs and upgrading existing vessel designs.
Recent deepwater development commitments have created a need for new generation drilling vessels and production facilities requiring a plethora of new technologies and systems to operate effectively in deep water and alien/harsh environments. These new technologies include riser tensioner development where direct acting cylinders are utilized.
Current systems as manufactured by Hydralift employ individual cylinders arranged to connect one end to the underside of the vessel sub-structure and one end to the riser string. These direct acting cylinders are equipped with ball joint assemblies in both the rod end and cylinder end to compensate for riser angle and vessel offset. Although this arrangement is an improvement over conventional wire rope systems, there are both operational and configurational problems associated with the application and vessel interface. For example, one problem is the occurrence of rod and seal failure due to the bending induced by unequal and non-linear loading caused by vessel roll and pitch. Additionally, these systems cannot slide off of the well bore centerline to allow access to the well. For example, the crew on the oil drilling vessel is not able to access equipment on the seabed floor without having to remove and breakdown the riser string.
The tensioner system as described in U.S. Pat. Nos. 6,530,430 and 6,554,072, both of which are incorporated herein by reference in their entirety, was an improvement over then-existing conventional and direct acting tensioning systems. Beyond the normal operational application to provide a means to apply variable tension to the riser, such a system provides a number of enhancements and options including vessel configuration and its operational criteria.
Such a tensioner system has a direct and positive impact on vessel application and operating parameters by extending the depth of the water in which such a system may be used and operational capability. In particular, such a system is adaptable to existing medium class vessels considered for upgrade by reducing the structure, space, top-side weight and complexity in wire rope routing and maintenance, while at the same time increasing the number of operations which can be performed by a given vessel equipped with such a tensioner system.
Additionally, such a tensioner system extends operational capabilities to deeper waters than other conventional tensioners by permitting increased tension while reducing the size and height of the vessel structure, reducing the amount of deck space required for the tensioner system, reducing the top-side weight, and increasing the oil drilling vessel's stability by lowering its center of gravity.
Moreover, such a tensioner system is co-linearly symmetrical with tensioning cylinders. Therefore, such a tensioner system eliminates offset and the resulting unequal loading that causes rapid rod and seal failure in some previous systems.
Such a tensioner is also radially arranged and may be affixed to the vessel at a single point. Therefore, such a tensioner may be conveniently installed or removed as a single unit through a rotary table opening, or disconnected and moved horizontally while still under the vessel.
Such a tensioner system further offers operational advantages over conventional methodologies by providing options in riser management and current well construction techniques. Applications of the basic module design are not limited to drilling risers and floating drilling vessels. Such a system further provides cost and operational effective solutions in well servicing/workover, intervention and production riser applications. These applications include all floating production facilities including, tension leg platform (T.L.P.) floating production facility (F.P.F.) and production spar variants. Such a system, when installed, provides an effective solution to tensioning requirements and operating parameters including improving safety by eliminating the need for personnel to slip and cut tensioner wires with the riser suspended in the vessel moon pool. An integral control and data acquisition system provides operating parameters to a central processor system which provides supervisory control.
However, such a tensioner system, as described in U.S. Pat. Nos. 6,530,430 and 6,554,072, has the drawback that the manifold therein requires at least two radial fluid bands, wherein at least one of the at least two radial fluid bands is in communication with each of the tensioning cylinders therein, so that individual control of each tensioning cylinder separately is not possible in such a tensioner system. In addition, the rod ends of the tensioning cylinders are required to communicate with flexjoint bearings, adding to the complexity and expense of such a tensioner system.
Hydraulic workover (HWO) units are conventionally rigged up either in a non-compensated fashion (rigged up and connected to the rig floor by pipe slips), or in a motion compensated system by using the drill rig's own compensation system, as shown, for example, in FIG. 1. For motion compensation, the HWO units can be rigged up in a tension lift frame assembly similar to the way coiled tubing injectors are rigged up. The tension lift frame may be connected to the top drive, as indicated at 100, and is motion compensated through the drill line's own compensation system. However, this leaves the HWO unit occupying valuable real estate above and/or on the rig floor, increases the overall height above the rig floor, which increases the danger potentially posed by objects that may fall from above the rig floor, and ties up the rig block.